The present invention relates to fluoroelastomers that have a low glass transition temperature, i.e. that have a glass transition temperature of xe2x88x9210xc2x0 C. or less and to a method of making them. More particularly, the present invention relates to (per)fluoroelastomers that are derived from a fluorinated olefin selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene, perfluoromethyl vinyl ether and mixtures thereof and one or more liquid perfluorovinylethers.
Fluoroelastomers and in particular perfluoroelastomers such as those described in xe2x80x9cModern Fluoropolymersxe2x80x9d, edited by John Scheirs, Wiley Science 1997, offer excellent protection against high service temperatures and are resistant to a wide variety of chemical reagents. Fluoroelastomers are generally divided in two main classes, namely those that are based on vinylidene fluoride (VF2) and those that do not contain VF2 and are based on perfluoromethylvinyl ether (PMVE), and tetrafluoroethylene (TEE) and/or chlorotrifluoroethylene (CTFE). Fluoroelastomers based on VF2 are disclosed in for example U.S. Pat. No. 5,696,216, U.S. Pat. No. 5,639,838, U.S. Pat. No. 4,418,186 and U.S. Pat. No. 4,619,983. VF2 based fluoroelastomers generally have a lower chemical and heat resistance as compared to fluoroelastomers that are based on TFE/CTFE and PMVE. Unfortunately, these latter fluoroelastomers have the disadvantage that the glass transition temperature is somewhat high thereby limiting their low temperature performance. Various approaches are known to lower the Tg of these fluoroelastomers.
Barney et al (J. Poly. Sci. A-1, 8, 1091-1098 (1970)) described the addition of Krytox(copyright) perfluoroalkyl polyether oils to lower Tg. However, these plasticizers can be extracted by solvents over time.
Uschold (U.S. Pat. No. 4,513,128) describes perfluoro-terpolymers consisting of tetrafluoroethylene, perfluoromethyl vinylether and at least 3 mol % of long chain vinylethers: 
n=3-30. The long-chain vinylether lowers the Tg significantly, however the incorporation is rather difficult. Therefore, one has to run the polymerization in perhalogenated solvents (e.g. R 113) or in aqueous emulsion polymerization in the presence of fluorinated alcohols. The disadvantages of these systems are: the use of perhalogenated solvents (e.g. R 113) is often critical due to environmental concerns and the removal of the fluorinated alcohols is often very difficult because they act as swelling agents.
U.S. Pat. No. 4,766,190 discloses the aqueous emulsion polymerization of tetrafluoroethylene, hexafluoropropylene and perfluorovinylethers of the formula CF2xe2x95x90CFOxe2x80x94(CF2CFXO)mxe2x80x94Rf wherein X is F or CF3, m is 1 to 50 and Rf is a perfluoroalkyl group. Although the disclosed fluoroelastomers have a low Tg, their method of making involves polymerization times of up to 28 hours (see Example 1), making their manufacturing expensive.
U.S. Pat. No. 5,891,974 describes the aqueous emulsion polymerization of tetrafluoroethylene and perfluorovinylethers:
CF2xe2x95x90CFxe2x80x94Oxe2x80x94(CF2)2(OCF2)nOCF3
n=1-5. However, the polymerization process disclosed to make these fluoroelastomers, is again in the order of 30 hours.
WO 99/48939 discloses peroxide curable perfluoroelastomers of a perfluoro-olefin, a perfluorovinyl ether, including a perfluoroalkyl vinyl ether and a halogen containing cure site. The perfluoroelastomers are produced by an aqueous emulsion polymerization.
It would now be desirable to improve the process of making fluoroelastomers that are based on a perfluorovinylether, and TFE and/or CTFE in particular to make the process less cumbersome and more cost effective. It would further be desirable to obtain fluoroelastomers with beneficial properties such as good cure properties and good mechanical and physical properties when cured.
The present invention provides a process of making a curable fluoroelastomer comprising the steps of:
pre-emulsifying one or more liquid perfluorovinylethers in water with the aid of a fluorinated emulsifier to obtain an aqueous emulsion, said liquid perfluorovinylether corresponding to the formula:
CF2xe2x95x90CF(O(CF2)n)m(OCF2)xORfxe2x80x83xe2x80x83(I)
xe2x80x83wherein n represents an integer of 1 to 6, In represents an integer of 1 to 3, x represents an integer of 0 to 3, with the proviso that the sum of x and m is at least 3 when n equals 1 and Rf represents a perfluoroalkyl group having 1 to 6 carbon atoms; and
copolymerizing said pre-emulsified liquid perfluorovinylethers with a gaseous fluorinated monomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene, perfluoromethylvinyl ether and mixtures thereof in the presence of one or more cure site component;
the amount of said one or more liquid perfluorovinyl ethers being sufficient so as to obtain a fluoroelastomer having a glass transition temperature of less than xe2x88x9210xc2x0 C.
It has been found that the above process allows for a substantially faster polymerization reaction than the polymerization processes disclosed in the prior art. In particular, it was found that by preemulsifying the liquid perfluorovinyl ether of the above formula prior to its copolymerization with the other comonomers, the polymerization time can be reduced and the incorporation is high. Fluoroelastomers having a low Tg and desirable physical and mechanical properties can thus be obtained in a convenient and cost effective way.
By the term xe2x80x9cliquid perfluorovinyletherxe2x80x9d is meant that the perfluorovinylether is generally present as a liquid at ambient conditions of temperature and pressure, i.e. at a temperature of 20xc2x0 C. and a pressure of 1 atm. By the term xe2x80x9cpre-emulsifiedxe2x80x9d in connection with the present invention is meant that the perfluorovinylether is emulsified in water with the aid of the fluorinated emulsifier prior to polymerization of the liquid perfluorovinylether. The term aqueous emulsion is to be understood as a liquid emulsified in water that generally has a milky appearance generally having a settling time of at least 1 hour. Such settling time is generally achieved using a non-telogenic fluorinated emulsifier other than a fluorinated polyether emulsifier. By the term xe2x80x9cboiling pointxe2x80x9d in connection with the present invention is meant a boiling point under ambient conditions, i.e. at a pressure of about 1 atm. By the term xe2x80x9cgaseousxe2x80x9d in connection with the present invention is meant that the respective compounds are present as a gas under ambient conditions of temperature and pressure, i.e. at a temperature of about 20xc2x0 C. and a pressure of about 1 atm.
Particularly preferred fluoroelastomers that can be obtained with the process of the invention are those that consist essentially of 15 to 45 mol %, preferably 20-35 mol % of repeating units derived from one or more liquid perfluorovinyl ethers of formula (I), 0.1 to 5 mol %, preferably 0.2 to 3 mol % especially preferred 0.4-2 mol % of cure site component, at least 50 mol % of units derived from a gaseous fluorinated monomer selected from the group consisting of tetrafluoroethylene, chlorotrifluoroethylene, perfluoromethylvinyl ether and mixtures thereof and 0 to 10 mol % of repeating units derived from further perfluorinated monomers other than said liquid perfluorovinyl ether and said gaseous fluorinated monomer.
An especially preferred fluoroelastomer that can be obtained with the process of the invention consists essentially of:
(a) 15 to 35 mol % of repeating units derived from one or more liquid perfluorovinyl ethers of formula (I),
(b) 0.1 to 5 mol % of cure site component,
(c) 1 to 15 mol % of repeating units derived from perfluoromethylvinylether,
(d) at least 50 mole % of repeating units derived from tetrafluoroethylene and/or chlorotrifluoroethylene;
and the sum of(a)+(b)+(c)+(d) adding up to 100 mol %. These polymers often show better product properties (e.g., lower hardness and lower compression sets) when compared to polymers having the same or very similar glass transition points but having no perfluoromethylvinylether incorporated.
The invention also provides a curable fluoroelastomer composition comprising the preferred and especially preferred fluoroelastomers set forth above and to a shaped fluoroelastomer article that can be obtained by curing such curable fluoroelastomer compositions.